In the field of gas turbine burners flame stabilization at the burner outlet is of high importance to obtain low emission and low combustion dynamics phenomena which can damage the combustor hardware. Usually the flow of fuel is delivered through injection nozzles within the burner system and combustion is achieved in the combustion chamber containing a combustion zone. A combustor of that kind is described in patent U.S. Pat. No. 6,152,726 (filing date 28 Nov. 2000). The known burner uses several injection channels for fuel supply. Currently gaseous fuel is used e.g. natural gas. A first fuel supply, which is used to inject the main portion of fuel is located in the swirler section, wherein nozzles are provided at the edges of the main swirler vanes. The secondary fuel supply may be located at a central lance, which extends coaxially with the main axis of the burner. This second fuel supply is optional and preferably used to fix the flame front at a specific location and to avoid high frequency fluctuations. A third fuel supply is used to ignite and maintain the flame front in the main combustion zone and located at the end of said outlet section, which comprises an annular rim protruding into said combustion zone, wherein said rim is provided with second fuel nozzles discharging fuel in a radial outward direction.
The known burner achieves low emission because most of the fuel is delivered at the swirler vane region which is capable to homogenously distribute the fuel in the airstream respectively to guarantee a good premix. Said central gas injection improves flame stability in a limited range of operation because at higher central gas flow rates the flame position moves an increases combustor dynamics. The third way to inject fuel by said external pilot nozzles improves stability by a diffusive type flame but increases also emissions which also limits the range of operation. At full load conditions the emissions need to be low and therefore the external pilot is fed by only a minor portion of fuel which leads to a smaller pilot flame region which is less effective in stabilizing the main flame. Especially in the outlet section of the burner between the main combustion zone and said rim of the outlet section the flow is highly turbulent while a shear layer develops between a high flow speed and regions of decreased flow speed. This shear layer develops between the diffusive flame type of the external pilot and the main combustion zone which reduces the stabilizing effect of the external pilot located at the outer rim of said outlet section for the main combustion zone flame front. To avoid the flame front of the main combustion zone to be extinguished the fuel flow of the external pilot has to be increased which results in higher NOX emissions. This effect is further versant by injection of air through mixing tube cooling air holes which reduces the fuel concentration on the shear layer further. To compensate this effect respectively to stabilize the combustion the fuel supply to the external pilot must be further increased which does not only increase NOX emissions but also lead to temperatures at the external rim of said outlet section which are not acceptable with regards to material properties of said outlet section.